Radio receiving system



s. MURcEK 2,228,084

RADIO RECEIVING SYSTEM Jan. 7, 1941.

Filed Feb. 18, 1939 I IN VENTOR Slam J/Vurce/r.

' A TTo NEY WITNESSES:

Patented Jan. 7, 1941 eras rarest RADM) RECEEVING SYSTEM sylvania Application February 1%, 1939, Serial No. 257,163

8 (Jlaims.

This invention relates to radio receiving systems and more particularly to systems where means are provided for eliminating the undesirable effect of static impulses or transient voltage surges.

Under the term static it is to be understood any disturbance which has the character of an oscillatory discharge usually of highly damped oscillations which set-up electromagnetic wave trains to which a receiving system responds together with the desired carrier frequency.

The successful elimination of the disturbing eifect of such static impulses whether they are natural phenomena due to atmospheric disturbances or are generated by various electrical apparatus has been the problem of many investigators in the past. The remedies suggested may involve various portions of a receiving system. The present invention contemplates the elimination of staticdisturbances in the receiving system at av point where the signal reaches the demodulator stage, and is about to enter the succeeding audio-frequency amplifier or a suitable translating device which may follow the demodulator.

The: primary object of the invention is the elimination of the effect. of static disturbance by means of cancellation of signal impulses having a predetermined frequency character. To this end means are provided whereby undesired si nals after detection become balanced-out due to the cancelling effect of similar signal components reacting in opposite phase to the original signal.

The circuit contemplated by the present invention is a dual channel transmission path or demodulator circuit in which signals of both channels are out of phase and in which one channel becomes operative only at certain'predetermined audio-frequencies. At such frequencies the outputs of both channels being combined in a manner to neutralize each other becomes efiectively cancelled and no signal reaches the portion of the system which succeeds the detector stage.

Analysis has shown that disturbances of the type referred to are usually higher audio-frequency modulations resulting in audio-signals predominantly in the high frequency portion of the audio-spectrum. These frequencies, while still being a part of the audio range, are outside of the useful portion of operation and to a large extent even outside of the high frequencies necessary for the reproduction of musical programs. Therefore, cancellation of signals at this extreme high frequency portion of the audio-range for instance between 3000 and 10000 cycles has little effect on the faithful rendition of average musical selections and may be sacrificed when thereby static surges are eliminated which mar the reproduction of the entire program.

A particular feature of the invention is the automatic cancellation of signals at a critical high audio-frequency and therebeyond, this frequency being predetermined and the system adjustable for a range of' frequencies wherein cancellation is desired.

Other features and advantages will be apparent from the following description of the invention, defined in particularity by the appended claims and taken in connection with the accompanying drawing in which:

Figure 1* shows a conventional receiving system incorporating in the demodulator portion the cancellation circuit in accordance with this invention, and

Fig. 2 shows a modification thereof for audiofrequency transmission'line circuits.

Referring to Fig; 1, the radio-frequency portion of the receiving system is merely schematica-lly indicated by a number of high-frequency amplifying stages in cascade arrangement. For the purpose of simplification, the type shown here is a tuned radio-frequency system. It is to be understood that any suitable type of high frequency receiving arrangement may as well be employed such for example as a superheterodyne type of receiver. The signal to be demodulated is impressed upon a coupling unit between the last high frequency amplifying stage and the detector, herein shown as a radio-frequency transformer I, having a primary winding 2 and a secondary winding 3.- The latter forms a divided input circuit between a centertap l which is grounded by the lead ii. The two halves of the secondary winding by virtue of the centertap and with respect thereof will have a signal voltage identical in character but opposite in phase by 180 between the two halves. Two separate channels are provided'for impressing the signal voltage from each half of thesecondary winding 3 to a separate demodulator comprising the dual diode rectifier tube 5. The upper half of the secondary winding 3 is connected by means of conductor 1 and coupling condenser 8 to the anode 9 of the tube 6, whereas the lower half of the secondary winding. 3 connects through conductor l0, and coupling condenser H to the second anode l2 of the tube 8. The cathode l3 cooperating with the anode I2 is grounded being thereby at an equi-potential. with respect to signal voltages between the two halves of the secondary winding 3. A load resistance I4 is provided between anode l2 and cathode 13 which is by-passed for radio frequencies by the condenser IS. The second portion of the tube 6 comprising the anode 9 and cathode I6 is similarly connected, the latter being grounded and a load resistance I1 is provided in the cathode-anode path, which is by-passed for radio-frequencies by the condenser I8. In the cathode-anode path 6 and I6 and in series with the resistor I1 is provided a tuned circuit, comprising the inductance l9 shunted by condenser 20. This tuned circuit is preferably enclosed in a shielding compartment schematically shown by the dotted lines connected to ground. The tuned circuit as stated before is eifectively in series between anode 9 and cathode I6, conductor 2| connects one terminal thereof with the anode 9 and conductor 22 the other terminal with the load resistance H. The latter is variable as shown by the arrow, the purpose of which shall be explained later.

The audio-frequency output of each diode rectifier which may be taken off from the anode terminal of each load resistance [4 and I1, respectively, is applied to the succeeding amplifying stage comprising the audio-amplifying tube 23, the latter feeds the succeeding stage which may be the final power output of the receiving system energizing a translating device such as a loud speaker. The amplifier tube 23 is of special construction, and is known as a dual grid mixer. In practical application this type of tube is commonly referred to as the type 6L7 having two separate control grids shielded from each other. This design permits each control grid to act independently on the electron stream and provides dual control in a single stage. One of the control grids 24 receives excitation voltage from the output .circuit including the anode 9 of the tube 6, being connected thereto by means of coupling condenser 25. The second control grid 21 is similarly connected through coupling condenser 28 to the anode [2 of the tube 6. Input circuit 26 and resistors 29 are provided for the grids 24 and 2i, respectively. Both resistors 23 and 29 forming the control grid circuits of the tube 23 return through suitable filter resistors 26' and 29, respectively, to a potential supply, not shown here, which may be a battery supplying the necessary bias-potential between cathode 30 of the tube 23 and the two control grids 24 and 21. The screen grid electrode 3! of the tube 23, properly by-passed by condenser 32 and filter resistor 33, terminates also at the power supply for the required operating voltage. The anode 34 of the tube 23 terminates in the anode resistor 35 and is supplied with anode voltage through filter resistor 36 which is by-passed by the condenser 31. The output of the tube 23 is coupled by means of coupling condenser 38 to the succeeding amplifier stage indicated here by a conventional square.

The operation of the circuit of Fig. 1 shall be explained hereafter. It is assumed that the high frequency stages are tuned to receive a certain high-frequency carrier which may be modulated by speech or music extending in frequency range between the lowest audible frequencies up to approximately 10,000 cycles. A signal voltage of a certain magnitude will appear in both halves of the secondary winding 3 and 4 of the coupling transformer and this voltage is applied to the respective diode rectifiers. For the sake of easier understanding, let us consider the lower half of the secondary winding which feeds the diode between anode l2 and cathode l3. This will be referred to as the signal voltage rectifier. The radio frequency voltage supplied by the lower line of the secondary winding will be rectified and the audio-frequency component appearing across the load reistance I4 is the modulation frequency which is then appplied to the grid 21 through the coupling condenser 28 as previously described. Under normal operating conditions the required operating potentials for the electrodes of the tube 23 being supplied from a suitable power supply, the audio voltage energizing the grid electrode 21 will be amplified and further conducted to the succeeding stage. The tube, 23 as stated before, is a dual grid device and the condition of its second grid electrode 24 also influences its operation. As can be seen following the diagram, the second grid electrode 24 is connected in a manner similar to the first control grid 21 to the output of the second diode-rectifier of the composite detector tube. It is also clear that voltages in the output circuits of the rectifiers are opposite in phase. Therefore, if these two voltages are simultaneously applied to the two control grids, cancellation of signal will result inasmuch as when one grid tends to swing positive with respect to its cathode, the second grid will tend to swing negative. In order that this condition of cancellation should. appear only at a certain frequency of the audio-signal which in this particular case is to be the high frequency portion of the audio spectrum, a tuned filter circuit is provided between anode and cathode of the control rectifier. The filter circuit comprises an inductance l9 and capacitance which is sharply tuned to approximately 8000 cycles. The tuned circuit bemg parallel resonant will offer a man impedance to frequencies at and above 8000 cycles, resulting in increased signal voltage output of the control rectifier, whereas the frequencies below 8000 cycles, that is, at the desired audio-frequencies there will be practically no excitation voltage for the second grid 24 of tube 23. The amount of control voltage to cancel the'signal may be regulated by varying the load resistor ll of the control rectifier. In other words, the circuit is definitely arranged to cancel or neutralize certain audio frequency components of the modulated signal immediately after detection. If the two diode rectifiers were loaded in exactly the same manner, no signal would be found in the plate circuit of the mixer tube 23.

However, the balanced condition explained in the preceding paragraphs is destroyed, since the frequency response characteristics of the loading impedances are not identical, one being a pure resistance load, and the other a resonant circuit. Thus, the signal variations appearing in the plate circuit of the mixer tube are efiectively the differences between the peak potentials developed across the loading impedances.

tifiers provides greater ease incircuit shielding procedure, making the circuit quite stable.

Experimental tests have shown that interfering noises due to static or similar discharges result, upon rectification, in high frequency audiosignals in the neighborhood of 8000 cycles or above. By virtue-of the circuit arrangement and the action of the tube .23 all signal voltages at the critical frequency of 8000 cycles or more can be eifectively eliminated by means of cancellation in the grid circuit of the first audio-amplifier tube. The system is very flexible due to the fact that, depending upon the design of the tuned circuit comprising inductance l9 and capacitance 22:, any desired frequency .or frequency range may be selected as a controlling factor in the operation of the system. By suitable choice of certain constants, any desired audio-frequency response may be obtained and certain characteristics of an audio-amplifier corrected.

Referring to Fig. 2, the modification .of the invention applies to an audio-frequency transmission channel, for example, a telephone line. The input circuit is shown by transformer 39, the primary 40 of which connects to the incoming line and the secondary 4| forms the input circuit of an amplifier tube 42 between grid 43 and cathode 44. The output circuit of the amplifier 42 includes between anode 45 and cathode 44, the primary winding 46 of a coupling transformer 41. The secondary winding 48 of the latter provides the divided input circuit being centertapped at 49 and connected by means of conductor 50 to the cathode circuit which is at ground potential for audio-frequency currents in the system. One half .of the secondary winding 48 is connected to the grid 5| of the amplifier tube 52, Whereas the other terminal of the second half of the secondary winding 48 is connected to the grid 5| of the control tube 52'. The cathodes 53 and 53' of the tubes 52 and 52, respectively, return to the cathode heater supply represented by the battery A. The anode 54 .of the tube 52 connects to the primary winding 55 of an output transformer 56. The anode 54' of the. control tube 52' is connected to the anode 54 through a tuned circuit comprising the inductance 5'! paralleled by a capacity 58. The necessary power supply for the anode circuit and grid circuit for the tubes is represented by the batteries B and C, respectively.

The operation of this circuit is based also upon the concept of phase balancing. The voltage in the grid circuit between each half of the secondary winding 48 is opposite in phase and the two plate circuits are connected together whereby the output voltages cancel. The tuned circuit comprising the inductance 51 and the capacitance 58 serially connected in the anode return circuit of the tube 52 forms a filter arrangement whereby the value of the opposing voltage will depend upon the resonant characteristic of the tuned circuit. By suitable choice of constants this can be made to operate in a desired frequency range.

In Fig. 2, the resonant circuit or network is arranged to vofier a high impedance to the desired signals or variation, thus providing an unbalanced condition only for the desired signals. For all other conditions the circuit is in a balanoed condition, or nearly so, with the result that the amplifier output for these conditions is at a minimum.

I claim as my invention:

1. In a receiving system for modulated carrier frequency energy, a demodulationvcircuit including' a pair of unidirectional impedance devices, means for impressing carrier frequency energy in opposing .phase relation between input terminals of said devices, means for combining the modulation frequency output of said devices effectively in opposition, and means for controlling the conductivity characteristic of one of said devices in accordance with predetermined modulation frequencies whereby said opposition is effective only at said predetermined frequencies within the range of modulationfrequencies derived from said demodulator.

2. In a receiving system for modulated carrier frequency energy, a demodulation circuit including a pair of rectifying devices, means for impressing carrier frequency energy in opposing phase relation between input terminals of said devices, means for combining the modulation frequency output of said devices effectively in opposition, and means for controlling the conductivity characteristic of one of said devices in accordance with certain predetermined modulation frequencies whereby said opposition iseffective only at said predetermined frequencies within the range of modulation frequencies derived from said demodulator.

3. Ina receiving system for modulated carrier frequency energy, a demodulation circuit including a pair of diode rectifiers having anode and cathode electrodes means for impressing carrier frequency energy in opposing phase relation between anode and cathode of said rectifiers, means for combining the modulation frequency output voltage of said rectifiers effectively in opposition, and means for controlling the voltage output of one of said rectifiers in accordance with certain predetermined modulation frequencies whereby said opposition is effective only at said predetermined frequencies within the range of modulation frequencies derived from said demodulator.

4. In a receiving system for modulated carrier frequency energy, a demodulation circuit including a pair of diode rectifiers having anode and cathode electrodes, a divided input circuit for impressing carrier frequency energy in opposing phase relation between anode and cathode of said rectifiers, means for combining the modulationfrequency output voltage of said rectifiers effectively in opposition, tending to cancel each other and means for controlling the voltage output of one of said rectifiers in accordance with certain predetermined modulation frequencies whereby said cancellation of output of said rectifiers effective at said predetermined frequencies within the range of modulation frequencies derived from said demodulator.

5. In a receiving system for modulated high frequency energy, a modulation circuit including a pair of demodulator tubes, a divided input circuit supplying high frequency energy coupled to said demodulators, said circuit being so arranged that signal voltages therein have a phase angle difference of substantially an output circuit for each of said demodulators, means for electronically combining said output circuits to energize an amplifier, a resonant circuit associated with one of said output circuits, said resonant circuit having a high impedance at a predetermined frequency within the modulation frequencies supplied to said amplifier whereby the phase opposition of voltages in said output circuits tending to cancel the energization of said amplifier is effectively operative at said predetermined frequency.

6. In a receiving system for modulated high frequency energy, a signal voltage amplifier, an output circuit therefor comprising a transformer having a primary and a secondary winding, a center-tapped connection for said last mentioned Winding, a pair of signal rectifiers each having an anode and a cathode, a load impedance between cathode and anode of each of said rectifiersya conductive connection between said center-tap and said cathodes, a signal input path for one of said rectifiers including its anode and one terminal of said secondary winding; a signal input path for the other said rectifiers including its anode and the other terminal of said secondary winding, individual output circuits for said rectifiers between anodes and cathodes respectively, one of said output circuits including a tuned circuit resonant to a predetermined frequency within the modulation frequency output of said rectifiers.

7. In a receiving system for modulated high frequency energy, a signal voltage amplifier, an output circuit therefor comprising a transformer havinga primary and a secondary winding, a center-tapped connection for said last mentioned Winding, a pair of signal rectifiers each having an anode and a cathode, a load impedance between cathode and anode of each of said rectiflers, a conductive connection between said center-tap and said cathodes, a'signal input path for one of said rectifiers including its anode and one terminal of said secondary winding, a signal input path for the other of said rectifiers including its anode and the other terminal of said secondary winding, individual output circuits for said rectifiers between anodes and cathodes respectively, one of said output circuits including a tuned circuit comprising an inductance and capacity in parallel therewith.

8. In a system for suppressing static disturbances and similar transient impulses afiecting the audio-frequency portion of a radio receiver, a dual signal channel, a rectifier in each of said channels, a circuit for combining the audio-frequency component of said rectifiers in opposite phase relation whereby the output of one channel is tending to cancel the output of the other channel, means for controlling the effective output of one of said channels in accordance with a certain frequency within the audio-frequencies to be received, comprising a resonant circuit having a high impedance at said certain frequencies, said resonant circuit being effective in producing a signal voltage of such magnitude as to cancel the signal voltage output of the other of said channels.

SLAVO J. MURCEK. 

